With the world's known sources of high grade copper and nickel diminishing rapidly, great emphasis has been placed on discovering new sources of these metals. There is known to be located throughout various regions on the globe, large, deep lying deposits of copper in the form of low grade porphyry ores. A porphyry copper ore deposit is a copper deposit in which the copper-bearing minerals occur in disseminated grains and/or in veinlets through a large volume of rock. The term was introduced because some of the first large copper deposits that were mined in the western United States occurred in porphyritic granodiorite and quartz monzonite. Today, the term implies a large low-grade disseminated copper deposit in various host rocks such as schist, silicated limestone, and volcanic rocks; but, acid igneous intrusive rocks are usually in close association.
The deposits are typically large tonnage but low grade, having an average copper concentration of less than about 1 percent. Copper minerals found in these deposits usually are sulfides and most commonly are chalcopyrite. When such a deposit is of sufficiently high grade, and either outcrops on the surface or is sufficiently close to the surface, then the ore is mined by open pit methods and the copper minerals are separated from the gangue constituents by techniques such as flotation.
Deeply buried or very low grade copper porphyry deposits cannot be easily exploited. Recovering the copper values from such deposits presents many challenges. For example, conventional open pit mining is not available for such recovery for a number of reasons. First of all, the cost would be prohibitive. Secondly, because open pit mining scars the landscape, restrictions have been placed on the recovery of ores by such techniques.
It has been proposed to extract the copper from deeply buried porphyry deposits by in-situ leaching techniques. In-situ leaching is a well-known technique which has long been practiced; its origin can be traced as far back as the 15th century. With in-situ mining, a hole is drilled and a leach liquor is pumped down the hole into the ore containing the metal to be recovered. After the liquor has leached the metal values, the pregnant leach liquor is extracted to enable metal values to be recovered.
There are also massive sulfide deposits treatable by the present invention which are deep seated and contain discrete blebs of nickel sulfide, or copper sulfide or copper-nickel sulfide in association with iron sulfide. A representative list of minerals which can be treated to recover copper or nickel or both by the present invention includes: native copper, chalcocite, digenite, covellite, pentlandite, heazlewoodite, vaesite and violarite.
There are many prior art procedures for in-situ mining. Most of these procedures, however, involve rubblizing the ore which is to be leached by explosive methods.
The present invention involves leaching the copper in-situ (without rubblizing it) with a lixiviant containing very small oxygen bubbles admixed with an ammoniated leach liquor. The oxygen bubbles are produced by a sparger or mixing device. To be effective, the oxygen bubbles should be smaller than the fractures in the ore.
A two-phase lixiviant containing small sparger-produced oxygen bubbles is an important aspect of the invention. However, the broad concept of utilizing an ammoniated lixiviant containing small bubbles of oxygen to leach copper from ore formations in-situ is disclosed in U.S. Pat. No. 3,708,206 to Hard et al. However, prior to the present invention, a two-phase introduction of oxygen was unattractive for a number of reasons. Problems such as phase separation of oxygen prevented a two-phase system from being used efficiently. In attempting to bring a suitable dispersion of oxygen into a bore when employing an aqueous solution, numerous adverse conditions apply. For example, elaborate methods and/or equipment was thought to be necessary to obtain a stable dispersion of oxygen as a gas in an aqueous fluid.
Indeed, so severe were the problems associated with two-phase in-situ mining procedures that research in this area was discouraged. The problems associated with two-phase in-situ mining are severe because the dispersion of oxygen must be sufficiently well distributed and the bubbles of oxygen must be sufficiently small so that these may enter the pores or fracture apertures in the rock before phase disengagement can occur. Still further, the quantity of oxygen should be evenly distributed throughout an entire ore column which is being worked by the in-situ method. Prior to the present invention, from the standpoint of complexity, economics and utilization of oxygen these considerations have made it almost intolerable to use oxygen as a gas dispersed in a liquid.
At this point it should be noted that the system disclosed in Hard et al. U.S. Pat. No. 3,708,206 involves recovering metals from porous rock such as sandstone located at depths close to the surface (300-500 ft.). The present process on the other hand is directed toward recovering metals from deep, hard rock formations of low porosity.
With a section of core material taken from a leaching interval of a typical deep lying porphyry copper ore, the copper is found primarily within the fractures. The fractures from which the copper is leached may be very small in size. Indeed, with the process of the present invention, copper can be recovered from fractures that are only 30 microns to 300 microns in width.
When the present invention is practiced, it is not necessary to disturb the deposit by blasting. Indeed, it is believed that the present invention is the only practical process presently known in which copper can be leached economically from deep lying deposits by in-situ mining techniques without rubblizing the ore. That copper can be leached from deep-lying porphyry ores without disturbing the ore is remarkable when the nature of the ore being leached is considered.
Another significant advantage of the process of the present invention is that the copper can be mined economically from deep lying porphyry deposits without any significant environmental impact. For example, with the present invention, there are no subsidence problems. Furthermore, the only alteration on the land surface is the presence of a few buildings and pumps which can be removed after the copper and/or nickel has been mined.